The invention relates to agricultural or industrial vehicles and particularly to speed control of such utility vehicles. Particularly, the invention relates to speed control of a utility vehicle that incorporates a hydrostatic transmission as an operator-controlled speed-adjusting component of the vehicle drive train.
Agricultural or industrial utility vehicles typically utilize a drive train having an engine driving a hydrostatic transmission that drives a final drive transmission or range transmission of the vehicle. The final drive transmission drives at least one wheel. JOHN DEERE Series 4000 tractors, available from John Deere Commercial Products, include such drive trains. The vehicle speed is typically operator-modulated by changing the drive ratio of the hydrostatic transmission via movement of a foot pedal. The drive ratio is changed by changing the angle of a swashplate of a variable displacement pump of the hydrostatic transmission.
Prior utility vehicles implement speed control or cruise control by various methods of setting and holding the swashplate angle of the variable displacement pump. In hydrostatic transmissions that are operator-modulated by mechanical displacement controls, cruise control devices have included electromagnets or friction plates to hold levers or pedals at a selected position corresponding to the desired speed. Electronically controlled hydrostatic transmissions usually hold the displacement of the variable displacement pump by maintaining a set electrical current to electro-hydraulic proportional pressure reducing valves which are used to control the swashplate angle.
In some hydrostatic transmission speed control systems, the vehicle speed is controlled by maintaining a constant energizing current set point to the proportional valves of the hydrostatic transmission. These systems are referred to as xe2x80x9copen loopxe2x80x9d systems, i.e., there is no mechanical feedback of the swashplate position. Hydrostatic transmission pumps that lack mechanical feedback control of the swashplate position cannot maintain constant ground speed for all conditions, simply by maintaining a level of electrical energizing current to the proportional valves. For example, increased load on the vehicle will result in a reduced speed due to engine speed changes or transmission efficiency changes.
In some other systems, swashplate mechanical position feedback control is provided. In these systems, the loads that tend to change the pump displacement by changing the swashplate angle are countered by swashplate mechanical position feedback control, so ground speed can be maintained by increasing or decreasing control current to the proportional valves in response to the mechanical position feedback control. However, this system provides a control loop that only maintains swashplate angle. The ground speed of the vehicle cannot be ensured by the control loop, as the control loop does not compensate for engine speed changes or transmission efficiency changes caused by load changes.
The present inventors have recognized the desirability to implement speed control in a hydrostatic transmission whereby swashplate mechanical position feedback control is not require. The present inventors have recognized the desirability of providing a speed control system, which is minimally affected by engine speed changes, or transmission efficiency changes caused by load changes. The present inventors have also recognized the desirability of providing an effective method of increasing or decreasing the speed set point for both slow and fast vehicle speeds.
This invention implements a speed control or cruise control system for a utility vehicle that is speed-modulated by a hydrostatic transmission. The speed control system includes a controller, such as a microcontroller having an input for receiving a selectable set speed signal, and an output for sending a speed control signal to at least one proportional valve of the hydrostatic transmission servo control system. The vehicle ground speed is continuously monitored by a speed sensor operatively associated with a rotating part in the vehicle drive train that rotates in proportion to vehicle ground speed, to create a ground speed signal. The selectable set speed signal can be input to microcontroller memory by driving the vehicle at a desired ground speed and then activating a speed set switch. By activating the speed set switch, the speed sensor inputs the current ground speed to the microcontroller memory. The speed control algorithm of the microcontroller thereafter compares the set speed to the ground speed signal from the speed sensor and corrects the control signal to the proportional valve to correct hydrostatic transmission speed output. Additionally, a speed increase/decrease switch is provided to manually increment the set speed, by an amount proportional to the set speed.
The hydrostatic transmission includes a variable displacement pump providing variable volume flow rate of a pressurized hydraulic fluid. The pump has an angularly adjustable swashplate, pump displacement being set by the angle of the swashplate. At least one proportional control valve is operatively connected to the swashplate to change the angle of the swashplate, the control valve being signal-connected to the output of the microcontroller. A hydraulic motor receives the pressurized hydraulic fluid from the pump to rotate the hydraulic motor.
The hydraulic motor is operatively connected to the rotating part of the drive train, preferably a gear, in order to rotate the part by rotary power from the hydraulic motor. The speed sensor is arranged to sense rotation of the part, the sensor being signal-connected to the microcontroller to send a rotation speed signal, or a stream of pulses, to the microcontroller. The microcontroller controls the proportional control valve to change the angular position of the swashplate in response to a difference, calculated by the microcontroller, between the rotation speed signal and the set speed.
A method of controlling the speed of a utility vehicle is set forth. The method includes the steps of: continuously sensing the rotational speed of a rotating part in a transmission of the vehicle; operating the vehicle at a desired ground speed; at the desired ground speed, selecting the rotational speed as a set speed; and if the rotational speed of the rotating part differs from the set speed, changing the transmission output to diminish the difference.
The preferred embodiment speed control system of the invention uses speed control foot pedals with potentiometer sensors, a Hall effect sensor that measures the speed of a gear in the final drive of the transmission, on/off and selector switches, and a programmed microcontroller having a speed control algorithm and memory means.
The operation of the preferred embodiment cruise control function is as follows: an on/off switch is positioned to activate the use of the cruise control function in the microcontroller software. Using the foot pedal controls on the vehicle, the operator drives the vehicle to the speed desired for the specific task. Once at the desired ground speed, a second switch is momentarily depressed and the microcontroller records the speed of a rotary part, e.g., a gear, present in the transmission that rotates proportionally to ground speed, as a set speed. The set speed is sensed by the Hall effect pulse pickup unit located adjacent to the rotating part, and is recorded in memory in the microcontroller. The current being supplied to proportional valves used to control the swashplate angle of the variable displacement pump is recorded in the memory of the microcontroller at the same time. Using the recorded current as a starting point, the control current to the proportional valves that control transmission output is modulated via a control algorithm to maintain the set speed of the vehicle. The algorithm monitors the speed signal from the Hall effect pulse pickup unit and increases or decreases the control current to the proportional valves using the error between the set speed and the actual speed signal from the pulse pickup unit.
While in cruise control mode, the set speed can be incremented up and down in steps by depressing momentary switches. For each depression of the switch, the microcontroller responds by changing the set speed by a percentage of the current set speed. This percentage is adjustable in the microcontroller software. Changing the speed by a percentage provides small speed changes when the vehicle set speed is slow and larger speed changes at higher vehicle set speeds. This is an advantageous feature of the invention. Slow operations generally require small speed adjustments, but larger speed adjustments are typically desired while transporting at higher speeds on the road.
The invention provides a cruise control for hydrostatic transmissions that does not require swashplate position feedback. By using a transmission speed signal to represent ground speed, and the speed signal to create a set point speed, and using feedback control, the set speed and feedback control are independent of engine speed changes and transmission efficiency changes caused by vehicle load changes.
Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and the embodiments thereof, from the claims and from the accompanying drawings.